Gastric adenocarcinoma remains the second most common cause of cancer-related death worldwide. The vast majority of gastric cancer evolves in the stomach in the setting of chronic atrophic gastritis usually in association with Helicobacter pylori infection. While the role of H. pylori as the proximate cause of gastric carcinogenesis is well established, the cellular basis of lineage changes that lead to development of preneoplastic metaplasia and progression to cancer remain unclear. The normal gastric fundic mucosa is assembled from a heterogeneous group of epithelial lineages responsible for the normal secretion of mucins, pepsinogen and HCl. Chronic injury associated with H. pylori infection leads to prominent changes in the composition of the gastric epithelia, with loss of parietal cells (oxyntic atrophy), expansion of surface cells (foveolar hyperplasia) and mucous cell metaplasia. Two metaplastic lineages are now acknowledged in the setting of oxyntic atrophy in humans: intestinal metaplasia (characterized by the presence of intestinal goblet cells in the gastric mucosa) and Spasmolytic Polypeptide Expressing Metaplasia (SPEM; characterized by presence of antral type mucous cells in the body of the stomach). However, Helicobacter infection in mice leads only to SPEM. Over the past 10 years, we have investigated the factors that lead to the development of SPEM in the face of oxyntic atrophy. Using a parietal cell directed protonophore, DMP-777, that induces acute loss of parietal cells, we have defined the dynamics of the induction of SPEM in rodents in the response to acute loss of parietal cells. Our recent lineage mapping studies in mice have demonstrated that SPEM arises, not from professional progenitor cells, but from transdifferentiation of mature Mist1-expressing chief cells into mucous cell metaplasia. All of these results support the hypotheses that loss of parietal cells from the gastric fundic mucosa induces the development of SPEM from transdifferentiation of chief cells and that intrinsic mucosal factors regulate the emergence of SPEM as the central step for initiation of gastric neoplasia. Thus, it is essential to understand the factors that elicit and control the emergence of SPEM as the critical initial event required for the development of gastric neoplasia. We will continue our studies of the origin of metaplasia through the prosecution of two specific aims: First, although our lineage mapping studies have determined that chief cells are giving rise to SPEM, suggesting that mature chief cells in general have the capacity to transdifferentiate into metaplasia. Nevertheless, it remains possible that subpopulations of chief cells exist that are the true cryptic progenitor cells for metaplasia. Therefore, we will seek to determine whether subpopulations of chief cells are responsible for development of SPEM by examining whether long- lived chief cell subpopulations may exist as putative quiescent progenitor cells. Furthermore, we will examine whether Lgr5-expressing chief cells in the gastric fundus may represent a quiescent progenitor cell population that could contribute to the development of SPEM. Second, we will seek to identify factors that may influence chief cell transdifferentiation into SPEM in vitro using novel conditionally immortalized cell culture models for mouse chief cells (ImChief) that we have developed during the previous funding period. In ImChief cells, we will determine whether over-expression of the chief cell differentiation-specific transcription factor Mist1 can promote further maturation of chief cell properties. In addition, we will determine whether loss of Mist1 promotes the development of metaplasia following induction of acute parietal cell loss in Mist1 KO mice and in targeted knockdown of Mist1 expression in ImChief cells. Finally, we will examine whether putative regulators of metaplastic transition, identified in our recent gene microarray analysis of mouse SPEM models, may promote chief cell transdifferentiation into SPEM when expressed in ImChief cells.))Through these studies we will gain fundamental insights into the cellular processes that lead to the induction of metaplasia as the critical initiating step for gastric carcinogenesis. )